Disease-associated MYH11 tail variants disrupt vascular smooth muscle myosin II filament dynamics and cell mechanics

Smooth muscle myosin II (SMII), encoded by MYH11, is the principal motor driving vascular smooth muscle cell (VSMC) contraction. How disease-associated MYH11 variants alter SMII behavior in living cells remains poorly understood. We directly visualized two human MYH11 tail variants in VSMCs: pathogenic K1256del and a variant of uncertain significance A1839V, identified in a patient with severe aortoiliac aneurysmal disease. Both variants reduced SMII filament assembly and VSMC contractile output, but with distinct severity. A1839V predominantly retained filamentous organization and displayed partial agonist-induced stabilization, consistent with an intermediate cellular phenotype. In contrast, K1256del severely disrupted filament architecture and behavior, including reduced focal adhesion organization, increased SMII network flow and accelerated filament exchange. Furthermore, K1256del abolished the filament stabilization response to induced contraction. These findings identify failure of SMII filament stabilization as a cellular mechanism linking MYH11 variation to impaired VSMC mechanics. More broadly, they show that direct live-cell phenotyping can distinguish variant-specific defects in the contractile machinery, providing a scalable functional strategy to complement genetic classification and prioritize MYH11 variants for deeper mechanistic and in vivo study.